If the fence is restricting access to an area of common property then the OC can demand its removal. Indeed if it would prevent members of the OC from observing any hazard or other maintenance requirement on that patch of common property, the OC should insist on the fence being removed.
Our OC, a set of townhouses, had several instances of people having enclosed areas of common property adjacent to their units. Although nobody else would be likely to use those bits of common property except the adjacent unit owner and although the adjacent unit owner was permitted, even encouraged, to look after the garden in those areas, we (the executive committee at the time) nonetheless insisted on the fences being removed, first with a nice letter and later with a rules infringement notice when the nice letter was ignored.
Two of the units changed hands soon after and the new owners had been made aware of the situation during the sale process. The new owners removed the fences. In a third instance we went to the Tribunal and eventually the unit owner complied after some mediation and firm and difficult but friendly and polite discussion made it clear that they really did have to comply.